LibreOffice does not do two-sided printing on HP OfficeJet Pro 7740. I loader HPLIP for Linux version 3.16.10 (the last version which fully supports the 7740). The printer is connected by USB cable. On both Ubuntu MATE 17.10 and 18.04, LibreOffice printer setting show 2-sided long-edge duplex printing. The HP Device Manager shows 2-sided long-edge duplex printing. The printer properties at print time show 2-sided long-edge duplex printing. Then LibreOffice prints only 1 side.
By accident, I discovered that Chrome does print 2-sided. This seems to indicate that the problem is in LibreOffice.